In connection with polarization, compensation and retardation layers, films, or plates described in the present application, the following definitions of terms are used throughout the text.
The definition of a “thin” optical film is related to the wavelength of light: a thin optical film is that having a thickness comparable with a half of the wavelength of light in the working interval of optical spectrum.
The term optical axis refers to a direction in which the propagating light does not exhibit birefringence. The optical properties of an optical compensation film are described by the refractive index ellipsoid, with the refractive indices nx, ny, and nz in the directions of x, y and z axes, respectively. The in-plane x and y axes are mutually orthogonal and are both orthogonal to the vertical z axis.
Liquid crystals are widely used in electronics as optical display elements. In such display systems, a liquid crystal cell is typically situated between a pair of polarizer and analyzer plates. The incident light is polarized by the polarizer and transmitted through a liquid crystal cell, where it is affected by the molecular orientation of the liquid crystal that can be controlled by applying a bias voltage across the cell. Then, the thus altered light passes through the second (output) polarizer that is called analyzer. By employing this scheme, the transmission of light from any external source, including ambient light, can be controlled. The energy required to provide for this control is generally much lower than that required for controlling the emission from luminescent materials used in other types of displays such as cathode ray tubes (CRTs). Accordingly, liquid crystal technology is used in a number of electronic imaging devices, including (but not limited to) digital watches, calculators, portable computers, and electronic games, for which small weight, low power consumption, and long working life are important.
The contrast, colour reproduction (colour rendering), and stable grey scale intensity gradation are important quality characteristics of electronic displays employing liquid crystal technology. The primary factor determining the contrast of a liquid crystal display (LCD) is the propensity for light to “leak” through liquid crystal elements or cells, which are in the dark or “black” pixel state. In addition, the optical leakage and, hence, the contrast of an LCD also depend on the direction from which the display screen is viewed. Typically, the optimum contrast is observed only within a narrow viewing angle range around the normal (α=0) to the display and falls off rapidly as the polar viewing angle α is increased. The viewing direction herein is defined as a set of the polar viewing angle (α) and the azimuthal viewing angle (β) as shown in FIG. 1 with respect to a liquid crystal display 1. The polar viewing angle α is measured from the display normal direction 2 and the azimuthal viewing angle β spans between an appropriate reference direction 3 in the plane of the display surface 4 and the projection 5 of viewing arrow 6 onto the display surface 4. Various display image properties such as the contrast ratio, color reproduction, and image brightness are functions of the angles α and β. In colour displays, the leakage problem not only decreases the contrast but also causes colour or hue shifts with the resulting degradation of colour reproduction.
LCDs are now replacing CRTs as monitors for television (TV) sets, computers (especially in notebook computers and desktop computers), central control units, and various devices, for example, gambling machines, electro-optical displays, (e.g., in watches, pocket calculators, electronic pocket games), portable data banks (such as personal digital assistants or mobile telephones). It is expected that the proportion of LCD television monitors with a larger screen size will also sharply increase in the nearest future. However, unless problems related to the effect of viewing angle on the colour reproduction, contrast degradation, and brightness inversion are solved, the replacement of traditional CRTs by LCDs will be limited.
The type of optical compensation required depends on the type of display used in each particular system. In a normally black display, the twisted nematic cell is placed between polarizers whose transmission axes are parallel to one another and to the orientation of the liquid crystal director at the rear surface of the cell (i.e., at the cell side that is away from the viewer). In the unenergized state (zero applied voltage), normally incident light from the backlight system is polarized by the first polarizer and transmitted through the cell with the polarization direction rotated by the twist angle of the cell. The twist angle is set to 90 DEG so that the output polarizer (analyzer) blocks this light. Patterns can be written in the display by selectively applying a voltage to the portions of the display that are to appear illuminated.
However, when viewed at large angles, the dark (unenergized) areas of a normally black display will appear bright because of the angle-dependent retardation effect for the light rays passing through the liquid crystal layer at such angles, whereby off-normal incident light exhibits an angle-dependent change in the polarization. The contrast can be restored by using a compensating element, which has an optical symmetry similar to that of a twist cell but produces the reverse effect. One method consists in introducing an active liquid crystal layer containing a twist cell of the reverse helicity. Another method is to use one or more uniaxial compensators. These compensation methods work because the compensation element has the same optical symmetry as that of the twist nematic cell: both are made of uniaxial birefringent materials having an extraordinary axis that is orthogonal to the normal light propagation direction. These approaches to compensation have been widely utilized because of readily available materials with the required optical symmetry.
Thus, the technological progress poses the task of developing optical elements based on new materials with desired controllable properties. In particular, important optical elements in modern visual display systems are optically anisotropic films with optical characteristics optimised for use in a particular display module.
Various polymeric materials are known in the prior art, which are intended for use in the production of optically anisotropic films. Films based on these polymers acquire optical anisotropy through uniaxial extension and coloration with organic or inorganic (iodine) dyes. Poly(vinyl alcohol) (PVA) is among polymers that are widely used for this purpose. However, a relatively low thermal stability of PVA based films limits their applications. PVA based films are described in greater detail in the monograph Liquid Crystals-Applications and Uses, B. Bahadur (ed.), World Scientific, Singapore-New York (1990), Vol. 1, p. 101.
Organic dichroic dyes constitute a new class of materials currently gaining prominence in the manufacture of optically anisotropic films with desirable optical and working characteristics. Films based on these materials can be obtained by applying an aqueous liquid crystal (LC) solution of supramolecules containing dye molecules onto a substrate surface, with the subsequent evaporation of water.
A hydrophobic-hydrophilic balance of molecules of polycyclic organic compounds makes them soluble in water and stimulates their self-assembly into supramolecules. Organic compounds in water form a colloid system or lyotropic liquid crystal, where molecules aggregate into supramolecules and these supramolecules represent kinetic units of the colloidal system (see, P. I. Lazarev, M. V. Paukshto, “Multilayer optical coating,” U.S. 2004/0233528 (2004)). Spectral characteristics and rheological properties of materials (see, V. Nazarov, L. Ignatov, K. Kienskaya, “Electronic Spectra of Aqueous Solutions and Films Made of Liquid Crystal Ink for Thin Film Polarizers,” Molecular Materials, Vol. 14, No. 2, pp. 153-163 (2001); S. Remizov, A. Krivoshchepov, V. Nazarov, A. Grodsky, “Rheology of The Lyotropic Liquid Crystalline Material for Thin Film Polarizers,” Molecular Materials, Vol. 14, No. 2, pp. 179-190 (2001)) indicate strong tendency of these molecules to aggregate, even in diluted aqueous solutions, with formation of supramolecules with columnar structure. Columnar structure is specific for flat shaped molecules grouped in “face-to-face” fashion with hydrophobic molecular planar cores of aromatic conjugated bond system stacked on each other inside of the supramolecule core and the hydrophilic peripheral groups exposed to water. Water provides the medium for electrostatic interaction and mutual alignment of supramolecules with resulting lyotropic liquid crystal structure of certain symmetry at certain level of aggregates concentration. Formation of supramolecules starts at low concentration of amphiphilic compounds in the water. There are two types of data that can be used as a basis for previous statement which are (1) optical spectra of molecular compounds that are building block of supramolecules, and (2) light scattering data that correlate with size of aggregates that are present in the system.
The applied films are rendered anisotropic either by preliminary mechanical orientation of the substrate surface or by post-treatment using external mechanical, electromagnetic, or other orienting forces applied to the LC film material on the substrate.
Liquid crystal properties of dye solutions are well known. In recent years, use of liquid crystals based of such dye solutions for commercial applications such as LCDs and glazing coatings has received much attention.
Dye supramolecules form lyotropic liquid crystals (LLCs). Substantial molecular ordering or organization of dye molecules in the form of columns allows such supramolecular LC mesophases to be used for obtaining oriented, strongly dichroic films.
Dye molecules forming supramolecular LC mesophases possess unique properties. These dye molecules contain functional groups located at the periphery, which render these molecules soluble in water. Organic dye mesophases are characterized by specific structures, phase diagrams, optical properties, and solubility as described in greater detail in: J. Lydon, Chromonics, in Handbook of Liquid Crystals, Wiley VCH, Weinheim (1998), Vol. 2B, p. 981-1007 (see also references therein).
Anisotropic films characterized by high optical anisotropy can be formed from LLC systems based on dichroic dyes. Such films exhibit the properties of so-called E-type polarizers (due to the absorption of light by supramolecular complexes). Organic conjugated compounds with the general molecular structure similar to that of dye molecules, but exhibiting no absorption in the visible spectral range, can be used as retarders and compensators.
Retarders and compensators are the films possessing phase-retarding properties in the spectral regions where the optical absorption is absent. The phase-retarding or compensating properties of such films are determined by their double refraction also known as birefringence (Δn):Δn=|no−ne|,
which is the difference of the refractive indices for the extraordinary wave (ne) and the ordinary wave (no). The ne and no values vary depending on the orientation of molecules in a medium and on the direction of light propagation. For example, if this direction coincides with the optical or crystallographic axis, the ordinary polarization is predominantly observed. If the light propagates in the perpendicular direction or at some angle to the optical axis, the light emerging from the medium will separate into extraordinary and ordinary components.
It is also important to note that, in addition to the unique optical properties, the films based on organic aromatic compounds are characterized by a high thermal stability and radiation resistance (photostability).
Extensive investigations aimed at the development of new methods for fabricating dye-based films through variation of the film deposition conditions have been described in U.S. Pat. Nos. 5,739,296 and 6,174,394 and in published patent application EP 961138. Of particular interest is the development of new compositions of lyotropic liquid crystals by introducing modifying, stabilizing, surfactant and/or other additives in the known compositions, which improve the characteristics of LC films.
There is increasing demand for anisotropic films with improved selectivity in various wavelength ranges. Films exhibiting different optical absorption maxima over a wide spectral interval ranging from infrared (IR) to ultraviolet (UV) regions are required for a variety of technological applications. Hence, much recent research attention has been directed to the synthesis of new materials for the manufacture of isotropic and/or anisotropic birefringent films, polarizers, retarders or compensators (herein collectively referred to as optical materials or films) for LCD and telecommunication applications, such as (but not limited to) those described in P. Yeh, Optical Waves in Layered Media, New York, John Wiley &Sons (1998) and in P. Yeh and C. Gu, Optics of Liquid Crystal Displays, New York, John Wiley & Sons, (1999).
It has been found that ultrathin birefringent films can be fabricated using the known methods and technologies developed for the production of optically anisotropic films based on organic dye LLC systems. For example, the manufacture of thin, optically anisotropic crystalline films based on disulfoacids of the red dye Vat Red 14 has been described by P. Lazarev and M. Paukshto, Thin Crystal Film Retarders (in: Proceeding of the 7th International Display Workshops, Materials and Components, Kobe, Japan, Nov. 29-Dec. 1 (2000), pp. 1159-1160) In particular, such films can be obtained using cis- and trans-isomer mixtures of naphthalenetetracarboxylic acid dibenzimidazole:

This technology makes it possible to control the direction of the crystallographic axis of a film during the deposition and crystallization of LC molecules on a substrate (e.g., on a glass plate). The obtained films have uniform compositions and are characterized by high molecular and/or crystal ordering, with a dichroic ratio of approximately Kd˜28, which makes them useful optical materials, in particular, for polarizers, retarders, and birefringent films or compensators.
Thin birefringent films transparent in the visible spectral range have been also obtained based on disodium chromoglycate (DSCG):

The anisotropy of oriented films made of DSCG is not very high: a difference in the refractive indices Δn is in the visible range is approximately 0.1 to 0.13. However, the thicknesses of films based on DSCG can be varied over a wide range, thus making possible the preparation of films with desired phase-retarding properties despite low specific anisotropy characteristics of the material. These films are considered in greater detail in T. Fiske et al., Molecular Alignment in Crystal Polarizers and Retarders: Society for Information Display Int. Symp. (Boston, Mass., May 19-24 (2002), Digest of Technical Papers), pp. 566-569. The main disadvantage of many of these films is their dynamic instability, which leads to gradual recrystallization of the LC molecules and the resulting degradation of the optical anisotropy.
Other anisotropic film materials, based on water-soluble organic dyes, have been also obtained using the aforementioned technology; see, for example, U.S. Pat. Nos. 5,739,296 and 6,174,394 and European patent EP 0961138. However, such materials exhibit high optical absorption in the visible spectral range, which limits their use in applications requiring transparent birefringent films.
Still other anisotropic materials have been synthesized based on acenaphtho[1,2-b]quinoxaline sulfoderivatives having the general structural formula
where n is an integer in the range from 1 to 4; m is an integer in the range from 0 to 4; z is an integer in the range from 0 to 6; m+z+n≦10; X and Y are molecular fragments individually selected from the list including CH3, C2H5, OCH3, OC2H5, Cl, Br, OH, OCOCH3, NH2, NHCOCH3, NO2, F, CF3, CN, OCN, SCN, COOH, and CONH2; M is a counter ion; and j is the number of counter ions in the molecule; with a proviso that, when n=1 and SO3− occupies position 1, then m≠0 or z≠0.
Thus, there is a general need for films, which are optically anisotropic and sufficiently transparent in the spectral regions in which they are intended to operate. In particular, there is a need for such optical films transparent in the visible spectral range. It is therefore desirable to provide improved methods for the synthesis and manufacture of optically anisotropic films. It is also desirable to provide optical films resistant to humidity and temperature variations.